Although iOS does a reasonably good job of limiting which applications are allowed to continue running in the background and what they can do, sometimes even a legitimate background app can run away on you causing problems like excessive battery drain and possibly even unnecessary data usage. iOS only permits four types of third-party applications to run in the background at all: Voice-over-IP (VoIP) apps such as Skype, apps that play audio in the background, navigation apps, and Newsstand apps. Some other apps may also take advantage of the ability to continue running in the background to complete a task, but iOS itself limits these to a maximum of 10 minutes, after which the app will be summarily terminated by the operating system.
The most insidious of these are background apps that use the VoIP feature, since unlike audio and navigation apps, these don’t provide any obvious clues that they’re still running. Further, not all VoIP apps may be patently obvious; for example you may be surprised to know that Facebook qualifies as a VoIP app due to its new voice messaging features, and is therefore allowed to continue running in the background long after you’ve left the app. Facebook may in fact be the smoking gun in numerous recent reports of battery issues from iPhone users.
The good news is that you can easily terminate these background apps from the recently used apps list: Simply double-tap the Home button to bring the list up, and then tap and hold on any app shown in the list. The apps will start jiggling in the same way as reorganizing your home screen, and a red negative symbol will appear at the top-left corner of each icon. Simply tap on the red symbol to close the app, and it will stop running in the background. Be aware, however, that many VoIP apps will start to run in the background automatically if you restart your device, even though they are not shown in the recently used apps list, so if you think an app is causing problems, be sure to terminate it again after restarting your iPhone by running it at least once and then removing it from the recently used apps tray. Also keep in mind that the recently used apps tray is just that—a list of recently used apps; most of the apps shown here will not be running in the background at all unless they fit into one of the specific categories above.